Implement metadata-driven pgrx compilation on Linux & macOS

[theory]

The compliment to #42, but for pgrx distributions. As part of the design, develop an common interface (trait) shared by both packages, and a method to use a particualar implementation based on a string configuration:

• If the string is "pgxs", use the library from Implement metadata-driven PGXS compilation on Linux & macOS #42
• If the string is "pgxs", use the library here

The trait/interface should be supportable by other pipelines in the future, including

• Rust
• Go
• Perl
• Ruby
• npm

Done when:

• Can build two different pgrx extensions
• Can build on macOS arm64 and Linux amd64 & arm64
• Crate published/consumable for Design and implement basic CLI to build, test, and install #44